JP2012188731A - Low yield ratio and high strength hot-rolled steel sheet excellent in low-temperature toughness, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio and high strength hot-rolled steel sheet excellent in low-temperature toughness, suitable for steel pipe raw material.SOLUTION: The steel sheet has a composition containing, by mass, 0.03-0.10% C, 0.01-0.50% Si, 1.4-2.2% Mn, 0.005-0.10% Al, 0.02-0.10% Nb, 0.001-0.030% Ti, 0.05-0.50% Mo, 0.05-0.50% Cr, and 0.01-0.50% Ni so that Moeq defined with the following expression (1) falls within the range of 1.4 to 2.2%. The steel sheet has a structure containing a bainitic ferrite having ≤10 μm average grain diameter as a main phase and massive martensite having 1.4 to 15% area ratio and <5.0 aspect ratio as a second phase, wherein the size of the massive martensite is preferably ≤5.0 μm at a maximum and 0.5 to 3.0 μm on an average. The expression (1): Moeq(%)=Mo+0.36Cr+0.77Mn+0.07N.

Description

  The present invention relates to a low-yield-ratio high-strength hot-rolled steel sheet suitable as a material for spiral steel pipes or ERW steel pipes used in line pipes and a method for producing the same, and in particular, to prevent a decrease in yield strength after pipe forming. However, it relates to ensuring a low yield ratio and excellent low temperature toughness.

  In recent years, spiral steel pipes that are formed by spirally winding a steel sheet can be used to efficiently produce large-diameter steel pipes, and in recent years have come to be widely used as line pipes for transporting crude oil and natural gas. In particular, pipelines for long-distance transportation are required to have high transportation efficiency and have a high pressure, and there are many oil wells and gas wells in the cold region, which often leads to the cold region. For this reason, the line pipe used is required to have high strength and high toughness. Furthermore, from the viewpoint of buckling resistance and earthquake resistance, the line pipe is required to have a low yield ratio. The yield ratio of the spiral steel pipe in the longitudinal direction of the pipe hardly changes depending on the pipe making, and almost coincides with that of the hot-rolled steel sheet. Therefore, in order to reduce the yield ratio of a line pipe made of spiral steel pipe, it is necessary to lower the yield ratio of the hot-rolled steel sheet as the material.

In response to such a demand, for example, Patent Document 1 describes a method of manufacturing a hot-rolled steel sheet for a low-yield ratio high-tensile line pipe excellent in low-temperature toughness. The technology described in Patent Document 1 contains, by weight, C: 0.03-0.12%, Si: 0.50% or less, Mn: 1.70% or less, Al: 0.070% or less, and Nb: 0.01-0.05%. , V: 0.01 to 0.02%, Ti: 0.01 to 0.20% of a steel slab containing at least one kind is heated to 1180 to 1300 ° C, followed by rough rolling finish temperature: 950 to 1050 ° C, finish rolling finish temperature : Hot rolled under conditions of 760 to 800 ° C, cooled at a cooling rate of 5 to 20 ° C / s, started air cooling until reaching 670 ° C, held for 5 to 20s, then 20 ° C / It is cooled at a cooling rate of s or higher and wound at a temperature of 500 ° C. or lower to form a hot rolled steel sheet. According to the technique described in Patent Document 1, a tensile strength: 60 kg / mm ² or more (590 MPa or more) and a low yield ratio of 85% or less, and a fracture surface transition temperature: high toughness of −60 ° C. or less. It is said that rolled steel sheets can be manufactured.

  Patent Document 2 describes a method for producing a hot-rolled steel sheet for a high-strength, low-yield ratio pipe. The technology described in Patent Document 2 contains C: 0.02 to 0.12%, Si: 0.1 to 1.5%, Mn: 2.0% or less, Al: 0.01 to 0.10%, and further Mo + Cr: 0.1 to 1.5% The steel to be heated is heated to 1000-1300 ° C, the hot rolling is finished in the range of 750-950 ° C, and cooled to the coiling temperature at a cooling rate of 10-50 ° C / s, in the range of 480-600 ° C. It is a manufacturing method of the hot-rolled steel plate wound up. According to the technology described in Patent Document 2, without quenching from the austenite temperature range, ferrite is the main component, the area ratio is 1 to 20% martensite, and the low yield ratio is 85% or less. It is said that a hot-rolled steel sheet having a reduced yield strength after pipe making is obtained.

  Patent Document 3 describes a method for producing a low yield ratio electric resistance welded steel pipe excellent in low temperature toughness. The technique described in Patent Document 3 includes, in mass%, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.10%, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, the relationship between the contents of Mn, Si, P, Cr, Ni, Mo A slab having a composition containing Mneq satisfying 2.0 or more is hot-rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and retained in this temperature range for 10 minutes or more. Then, it is cooled to a temperature of less than 500 ° C. to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is formed into an electric-welded steel pipe. According to the technique described in Patent Document 3, it has a structure containing bainitic ferrite as a main phase and containing 3% or more martensite and, if necessary, 1% or more retained austenite, and has a fracture surface transition temperature of It is said that an ERW steel pipe having excellent low temperature toughness and high plastic deformation absorption ability can be produced at -50 ° C or lower.

  Patent Document 4 describes a low yield ratio high toughness thick steel plate. In the technique described in Patent Document 4, C: 0.03-0.15%, Si: 1.0% or less, Mn: 1.0-2.0%, Al: 0.005-0.060%, Ti: 0.008-0.030%, N: 0.0020-0.010% , O: Heated to a slab having a composition containing 0.010% or less, preferably 950 to 1300 ° C., and a reduction ratio in the temperature range of Ar 3 transformation point + 100 ° C. to Ar 3 transformation point + 150 ° C. is 10% or more, and finish rolling temperature After performing hot rolling at 800 to 700 ° C., start accelerated cooling within −50 ° C. from the finish rolling temperature, and after water cooling to 400 to 150 ° C. at an average cooling rate of 5 to 50 ° C./s, By air-cooling, a high tough steel plate with a low yield ratio having a mixed structure of ferrite having an average particle size of 10 to 50 μm and bainite in which 1 to 20 area% of island martensite is dispersed can be obtained. It is said.

JP 63-227715 A Japanese Patent Laid-Open No. 10-176239 JP 2006-299413 A JP 2010-59472 A

  However, in the technique described in Patent Document 1, it is necessary to control the cooling rate, the cooling stop temperature, and the like so as to be within a predetermined relatively fast cooling range, in particular, for producing a thick hot-rolled steel sheet. However, there is a problem that a large-scale cooling facility is required. Moreover, the hot-rolled steel sheet obtained by the technique described in Patent Document 1 has a problem that it has a structure mainly composed of soft polygonal ferrite and it is difficult to obtain a desired high strength. Moreover, in the technique described in Patent Document 2, a decrease in yield strength after pipe forming is still recognized, and there is a problem that a recent increase in steel pipe strength cannot be satisfied.

In addition, the technique described in Patent Document 3 has a problem in that it has not yet been able to stably secure excellent low-temperature toughness, which is a recent cold region specification, with a fracture surface transition temperature vTrs of −80 ° C. or lower. .
Moreover, in the thick steel plate obtained by the technique described in Patent Document 4, only a toughness of about −30 to −41 ° C. can be secured at the fracture surface transition temperature vTrs at the latest. There is a problem that can not be dealt with.

  The present invention solves the problems of the prior art, and does not require complex heat treatment, and without extensive modification of equipment, and is suitable for steel pipe materials, particularly for spiral steel pipes. It aims at providing the low yield ratio high toughness high strength hot-rolled steel plate which can prevent a fall. The term “high strength” as used herein refers to the case where the yield strength in the 30-degree direction from the rolling direction is 480 MPa or more and the tensile strength in the sheet width direction is 600 MPa or more, and “high toughness” is the Charpy impact test. When the fracture surface transition temperature vTrs is -80 ° C or less, the "low yield ratio" indicates a continuous yield type stress-strain curve and the yield ratio is 85% or less. And The “steel plate” includes a steel plate and a steel strip.

In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting steel pipe strength after pipe making and steel pipe toughness. As a result, the decrease in strength due to pipe making is caused by the decrease in yield strength due to the Bauschinger effect on the inner surface of the tube where compressive stress acts and the disappearance of yield elongation on the outer surface side where tensile stress acts. I found out.
Therefore, as a result of further research, the inventors have made a structure in which the structure of the steel sheet has fine bainitic ferrite as a main phase, and hard massive martensite is finely dispersed in the bainitic ferrite. As a result, it was conceived that a steel pipe having a low yield ratio of 85% or less and further excellent toughness can be obtained after pipe making, in particular, after strength reduction after spiral pipe making. . This is because, with such a structure, the work hardening ability of the steel plate material is improved, so that a sufficient increase in strength can be obtained by work hardening on the outer surface of the pipe during pipe making. In particular, it has been found that, after spiral pipe making, strength reduction can be suppressed, and toughness is significantly improved by finely dispersing massive martensite.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.10%, Si: 0.10-0.50%, Mn: 1.4-2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, Nb: 0.02 Containing 0.1% to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.01 to 0.50%, the balance Fe and unavoidable impurities, and bainitic ferrite The main phase and the second phase have a structure containing a bulk martensite with an aspect ratio of less than 5.0 in an area ratio of 1.4 to 15%, and the average grain size of the bainitic ferrite is 10 μm or less. Low yield ratio high strength hot rolled steel sheet with excellent low temperature toughness.
(2) In (1), the composition has the following formula (1)
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
(Here, Mn, Ni, Cr, Mo: Content of each element (mass%))
A low-yield-ratio high-strength hot-rolled steel sheet characterized by having a composition that satisfies the range of Moeq defined by 1.4 to 2.2%.
(3) In (1) or (2), in addition to the above composition, in addition to mass, Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less A low yield ratio high strength hot-rolled steel sheet characterized by containing more than seeds.
(4) In any one of (1) to (3), in addition to the above composition, the low yield ratio high strength hot-rolled steel sheet further contains Ca: 0.0005 to 0.0050% by mass%.
(5) The low yield ratio high strength heat according to any one of (1) to (4), wherein the bulk martensite has a maximum size of 5.0 μm or less and an average of 0.5 to 3.0 μm. Rolled steel sheet.
(6) When the steel material is subjected to a hot rolling process, a cooling process, and a winding process to form a hot rolled steel sheet, the steel material is in mass%, C: 0.03 to 0.10%, Si: 0.10 to 0.50% , Mn: 1.4 to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.02 to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.01 to 0.50%, a steel material having a composition consisting of the balance Fe and inevitable impurities, the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C, The heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar is subjected to a finish rolling at a temperature range of 930 ° C. or lower and finish rolling to be 50% or more to form a hot rolled steel sheet, The cooling process starts cooling immediately after finishing rolling, and cools to a cooling stop temperature in the temperature range of 600 to 450 ° C. at an average cooling rate of the center portion of the thickness of 5 to 30 ° C./s. In addition, the cooling from the cooling stop temperature to the winding temperature is cooled at an average cooling rate of 2 ° C./s or less, or the step is allowed to stay for 20 s or more in the temperature range from the cooling stop temperature to the winding temperature, A method for producing a high-strength hot-rolled steel sheet with low yield ratio and excellent low-temperature toughness, wherein the winding step is a step of winding at a surface temperature of a winding temperature of 450 ° C. or higher.
(7) In (6), the composition has the following formula (1)
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
(Here, Mn, Ni, Cr, Mo: Content of each element (mass%))
A method for producing a high-strength hot-rolled steel sheet having a low yield ratio, characterized in that the Moeq defined by the formula satisfies a range of 1.4 to 2.2%.
(8) In (6) or (7), in addition to the above composition, in addition to mass, Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less A method for producing a low-yield-ratio high-strength hot-rolled steel sheet characterized by containing at least a seed.
(9) In any one of (6) to (8), in addition to the above composition, the production of a low yield ratio high strength hot-rolled steel sheet characterized by further containing Ca: 0.0005 to 0.0050% by mass% Method.

  According to the present invention, particularly suitable as a material for spiral steel pipe, there is little decrease in strength after pipe forming, the yield strength in the direction of 30 degrees from the rolling direction is 480 MPa or more, and the tensile strength in the plate width direction is 600 MPa or more. , Low yield ratio high strength hot-rolled steel sheet with excellent low-temperature toughness, having a low yield ratio with a fracture surface transition temperature vTrs of -80 ° C or less and a yield ratio of 85% or less in the Charpy impact test. Without application, it can be manufactured easily and inexpensively, and has a remarkable industrial effect. In addition, according to the present invention, there is an effect that a line pipe laid by the reel purge method and an ERW steel pipe for a line pipe that requires earthquake resistance can be easily and inexpensively manufactured. Moreover, if the low-yield ratio high-strength hot-rolled steel sheet according to the present invention is used as a material, there is also an effect that a high-strength spiral steel pipe pile that becomes a building member having excellent earthquake resistance can be manufactured.

It is explanatory drawing which shows typically the relationship between the production | generation of massive martensite, and the secondary cooling in the cooling after hot rolling.

First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.03-0.10%
C is an element which precipitates as carbides and contributes to an increase in the strength of the steel sheet through precipitation strengthening and also contributes to an improvement in the toughness of the steel sheet through refinement of crystal grains. Further, C has an action of forming a solid solution in the steel, stabilizing austenite, and promoting the formation of untransformed austenite. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.10%, the tendency to form coarse cementite at the grain boundaries becomes strong, and the toughness decreases. For this reason, C was limited to the range of 0.03-0.10%. In addition, Preferably it is 0.04 to 0.09%.

Si: 0.10 to 0.50%
Si contributes to an increase in the strength of the steel sheet through solid solution strengthening and contributes to a reduction in the yield ratio through the formation of a hard second phase (for example, martensite). In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, if the content exceeds 0.50%, the generation of red scale becomes remarkable, and the appearance of the steel sheet deteriorates. For this reason, Si was limited to the range of 0.10 to 0.50%. In addition, Preferably it is 0.20 to 0.40%.

Mn: 1.4-2.2%
Mn is a solid solution that improves the hardenability of steel, promotes the formation of martensite, lowers the bainitic ferrite transformation start temperature, and contributes to the improvement of steel sheet toughness through refinement of the structure It is. In order to obtain such an effect, the content of 1.4% or more is required. On the other hand, the content exceeding 2.2% lowers the toughness of the weld heat affected zone. For this reason, Mn was limited to the range of 1.4 to 2.2%. In addition, from the viewpoint of stable production of massive martensite, it is preferably 1.6 to 2.0%.

P: 0.025% or less P dissolves and contributes to an increase in steel sheet strength, but at the same time lowers toughness. Therefore, in the present invention, it is desirable to reduce P as an impurity as much as possible, but up to 0.025% is acceptable. Preferably it is 0.015% or less. In addition, since excessive reduction raises refining cost, it is desirable to set it as about 0.001% or more.

S: 0.005% or less S forms coarse sulfide inclusions such as MnS in steel, causes cracking of slabs and the like, and decreases the ductility of the steel sheet. Such a phenomenon becomes remarkable when the content exceeds 0.005%. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.
Al: 0.005-0.10%
Al is an element that acts as a deoxidizer and is effective in fixing N that causes strain aging. In order to acquire such an effect, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.10%, the oxide in the steel increases and the toughness of the base metal and the welded portion decreases. Further, when a steel material such as a slab or a steel plate is heated in a heating furnace, a nitride layer is easily formed on the surface layer, which may increase the yield ratio. For this reason, Al was limited to 0.005 to 0.10% of range. In addition, Preferably it is 0.08% or less.

Nb: 0.02 to 0.10%
Nb dissolves in steel or precipitates as carbonitride, and has the effect of suppressing austenite grain coarsening and suppressing recrystallization of austenite grains. Make it possible. Moreover, it is an element which precipitates finely as carbide or carbonitride and contributes to an increase in strength of the steel sheet. During cooling after hot rolling, it precipitates as carbides or carbonitrides on dislocations introduced by hot rolling, acts as the core of γ → α transformation, promotes intragranular formation of bainitic ferrite, This contributes to the formation of fine massive untransformed austenite and, in turn, fine massive martensite. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, an excessive content exceeding 0.10% increases deformation resistance during hot rolling, which may make hot rolling difficult. An excessive content exceeding 0.10% leads to an increase in the yield strength of the main phase bainitic ferrite, making it difficult to ensure a yield ratio of 85% or less. For this reason, Nb was limited to 0.02 to 0.10% of range. In addition, Preferably it is 0.03-0.07%.

Ti: 0.001 to 0.030%
Ti fixes N as a nitride and contributes to the prevention of slab cracking, and has the effect of increasing the strength of the steel sheet by fine precipitation as a carbide. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 0.030%, the bainitic ferrite transformation point is excessively raised and the toughness of the steel sheet is lowered. For this reason, Ti was limited to the range of 0.001 to 0.030%. In addition, Preferably it is 0.005-0.025%.

Mo: 0.05-0.50%
Mo contributes to improving hardenability, attracts C in bainitic ferrite into untransformed austenite, and has an action of promoting martensite formation through improving the hardenability of untransformed austenite, Furthermore, it is an element that contributes to an increase in steel sheet strength by solid solution in steel and solid solution strengthening. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.50%, martensite is formed more than necessary, and the toughness of the steel sheet is lowered. In addition, Mo is an expensive element, and a large amount thereof causes an increase in material cost. For these reasons, Mo is limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10 to 0.40%.

Cr: 0.05-0.50%
Cr delays the γ → α transformation, contributes to improving hardenability, and has an action of promoting martensite formation. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if it exceeds 0.50%, defects tend to occur frequently in the weld. For this reason, Cr was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.20 to 0.45%.

Ni: 0.01-0.50%
Ni is an element that contributes to the improvement of toughness, in addition to promoting the formation of martensite, and further contributing to the improvement of toughness. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.50%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni was limited to the range of 0.01 to 0.50%. In addition, Preferably it is 0.30 to 0.45%.

The above-described components are basic components. In the present invention, the above-described components are contained within the above-described content range, and the following formula (1)
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
(Here, Mn, Ni, Cr, Mo: Content of each element (mass%))
It is preferable to adjust so that Moeq defined by the above satisfies the range of 1.4 to 2.2%.

  Moeq is an index representing the hardenability of untransformed austenite remaining in the steel sheet after passing through the cooling step. When Moeq is less than 1.4%, the hardenability of untransformed austenite is insufficient, and it transforms into pearlite or the like during the subsequent winding process. On the other hand, when Moeq exceeds 2.2%, martensite is generated more than necessary, and the toughness decreases. For this reason, Moeq is preferably limited to a range of 1.4 to 2.2%. If Moeq is 1.5% or more, the yield ratio is low and the deformability is further improved. For this reason, it is more preferable to set it as 1.5% or more.

In the present invention, one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less as a selection element, as necessary, within the range of the components described above. And / or Ca: 0.0005 to 0.0050%.
One or more selected from Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less
Cu, V, and B are all elements that contribute to increasing the strength of the steel sheet, and can be selected and contained as necessary.

  V and Cu contribute to the strengthening of the steel sheet through solid solution strengthening or precipitation strengthening, and B segregates at the grain boundary and improves hardenability. In order to obtain such effects, it is preferable to contain Cu: 0.01% or more, V: 0.01% or more, and B: 0.0001% or more. On the other hand, the content exceeding V: 0.10% decreases weldability, the content exceeding B: 0.0005% decreases the toughness of the steel sheet, and the content exceeding Cu: 0.50% decreases the hot workability. For this reason, when it contains, it is preferable to limit to Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to the control of the morphology of sulfides in which coarse sulfides are spherical sulfides, and can be contained as required. In order to acquire such an effect, it is preferable to contain Ca: 0.0005% or more. On the other hand, the content exceeding Ca: 0.0050% reduces the cleanliness of the steel sheet. For this reason, when it contains, it is preferable to limit to Ca: 0.0005 to 0.0050% of range.

The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, N: 0.005% or less, O: 0.005% or less, Mg: 0.003% or less, Sn: 0.005% or less are acceptable.
Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has the above-described composition, and further has a structure composed of bainitic ferrite as a main phase and a main phase and a second phase.

  Here, the main phase refers to a phase having an occupied area of 50% or more in area ratio. The main phase bainitic ferrite is a phase having a substructure with a high dislocation density, and includes acicular ferrite and acicular ferrite. The bainitic ferrite does not include polygonal ferrite having a very low dislocation density or quasi-polygonal ferrite with a substructure such as fine subgrains. In order to secure a desired high strength, it is necessary that fine carbonitride is precipitated in the bainitic ferrite as the main phase.

  The main phase bainitic ferrite has an average particle size of 10 μm or less. If the average crystal grain size exceeds 10 μm, the work hardening ability in a low strain region of less than 5% is insufficient, and the yield strength may be reduced by bending during spiral pipe making. Therefore, the average particle size of bainitic ferrite as the main phase is limited to 10 μm or less. By making the average particle size of the main phase fine, it is possible to ensure desired low-temperature toughness even when a large amount of martensite is contained.

  The hot-rolled steel sheet according to the present invention has a structure in which massive martensite having an aspect ratio of less than 5.0 is dispersed in an area ratio of 1.4 to 15% as the second phase. The massive martensite referred to in the present invention is martensite produced from untransformed austenite in the old γ grain boundaries or in the old γ grains in the cooling process after rolling. In the present invention, such massive martensite is dispersed between the old γ grain boundaries or the bainitic ferrite grains as the main phase and the bainitic ferrite grains. Martensite is harder than the main phase, and a large amount of movable dislocations can be introduced into the bainitic ferrite during processing, and the yield behavior can be a continuous yield type. Moreover, since martensite has a higher tensile strength than bainitic ferrite, a low yield ratio can be achieved. In addition, when the martensite is a massive martensite having an aspect ratio of less than 5.0, more movable dislocations can be introduced into the surrounding bainitic ferrite, which is effective in improving the deformability. When the martensite aspect ratio exceeds 5.0, it becomes rod-shaped martensite (non-agglomerated martensite) and the desired low yield ratio cannot be achieved, but it is acceptable if the area ratio relative to the total amount of martensite is less than 30%. it can. The bulk martensite is preferably 70% or more in terms of the area ratio of the total amount of martensite.

  In order to ensure such an effect, it is necessary to disperse massive martensite having an area ratio of 1.4% or more. If the bulk martensite is less than 1.4%, it becomes difficult to secure a desired low yield ratio. On the other hand, if the massive martensite exceeds 15% in area ratio, the low temperature toughness is remarkably lowered. For this reason, lump martensite was limited to the range of 1.4 to 15%. In addition, Preferably it is 10% or less.

  The size of the massive martensite is preferably 5 μm or less at the maximum and 0.5 to 3.0 μm on the average. When the bulk martensite size exceeds 3.0 μm on average, it becomes a starting point for brittle fracture or facilitates the propagation of cracks and lowers the low temperature toughness. On the other hand, if the average is less than 0.5 μm, the grains become too fine and the amount of movable dislocations introduced into the surrounding bainitic ferrite decreases. For this reason, the size of the massive martensite is preferably 5.0 μm or less at maximum and 0.5 to 3.0 μm on average. As for the size, 1/2 of the sum of the long side length and the short side length was defined as the “diameter”. The largest one of them was designated as “maximum”, and the value obtained by arithmetically averaging the “diameter” of each obtained grain was designated as “average”. The number of martensite to be measured is 100 or more.

Next, the preferable manufacturing method of this invention hot-rolled steel plate is demonstrated.
In the present invention, a hot-rolled steel sheet is obtained by subjecting a steel material having the above composition to a hot-rolling process, a cooling process, and a winding process.
In addition, the manufacturing method of the steel raw material to be used is not particularly limited, and the molten steel having the above-described composition is melted using a generally known melting method such as a converter or an electric furnace, and a continuous casting method or the like is used. It is preferable to use a steel material such as a slab by a generally known melting method.

The obtained steel material is subjected to a hot rolling process.
In the hot rolling process, the steel material having the above composition is heated to a heating temperature of 1050 to 1300 ° C., subjected to rough rolling to obtain a sheet bar, and then the sheet bar is subjected to cumulative reduction in a temperature range of 930 ° C. or less. Rate: It is set as the process which gives the finish rolling used as 50% or more, and makes it a hot-rolled steel plate.
Heating temperature: 1050-1300 ° C
The steel material used in the present invention essentially contains Nb and Ti as described above. In order to secure a desired high strength by precipitation strengthening, it is necessary to dissolve these coarse carbides, nitrides and the like once and then finely precipitate them. Therefore, the heating temperature of the steel material is 1050 ° C. or higher. If it is less than 1050 degreeC, each element will remain undissolved and desired steel plate strength will not be obtained. On the other hand, when the temperature exceeds 1300 ° C., the crystal grains become coarse and the steel sheet toughness decreases. For this reason, the heating temperature of the steel material was limited to 1050 to 1300 ° C.

The steel material heated to the heating temperature described above is subjected to rough rolling to be a sheet bar. The conditions for rough rolling need not be particularly limited as long as a sheet bar having a desired size and shape can be secured. Good.
The obtained sheet bar is then finish-rolled to obtain a hot-rolled steel sheet having a desired size and shape. The finish rolling is preferably rolling with a cumulative rolling reduction in a temperature range of 930 ° C. or lower: 50% or more.

Cumulative rolling reduction in the temperature range below 930 ° C: 50% or more Cumulative rolling reduction in the temperature range below 930 ° C is 50% or more for finer bainitic ferrite and fine dispersion of massive martensite. To do. If the cumulative rolling reduction in the temperature range of 930 ° C or lower is less than 50%, the rolling amount is insufficient, and fine bainitic ferrite as the main phase cannot be secured. In addition, dislocations that become precipitation sites such as NbC that promote nucleation of γ → α transformation are insufficient, intragranular formation of bainitic ferrite is insufficient, and massive untransformed γ to form massive martensite It cannot be dispersed finely and in large numbers. For this reason, the cumulative rolling reduction in the temperature range of 930 ° C. or lower in finish rolling is limited to 50% or more. The cumulative rolling reduction is preferably 80% or less. Even if the rolling reduction exceeds 80%, the effect is saturated, the occurrence of separation becomes significant, and the Charpy absorbed energy is reduced.

  In addition, it is preferable that the rolling completion temperature of finish rolling shall be 850-760 degreeC from viewpoints, such as steel plate toughness, steel plate strength, and rolling load. When the finishing temperature of finish rolling exceeds 850 ° C and becomes high, it is necessary to increase the reduction amount per pass in order to increase the cumulative reduction rate in the temperature range of 930 ° C or less to 50% or more. Increases load. On the other hand, when the temperature is lower than 750 ° C., ferrite is generated during rolling, resulting in coarsening of the structure and precipitates, and low temperature toughness and strength are reduced.

The obtained hot-rolled steel sheet is then subjected to a cooling step.
The cooling process starts cooling immediately after finishing rolling, and is the cooling stop temperature in the temperature range of 5 to 30 ° C / s at the average cooling rate at the center of the plate thickness and 600 to 450 ° C at the temperature at the center of the plate thickness. Primary cooling for cooling to a cooling temperature, and further cooling as a secondary cooling from the cooling stop temperature to the winding temperature at an average cooling rate of 2 ° C./s or less, or a temperature from the cooling stop temperature to the winding temperature. It is set as the process of retaining for 20s or more in a zone.

  Immediately after finishing rolling, cooling is preferably started within 15 s. The cooling rate of the primary cooling is an average cooling rate of 750 to 600 ° C. at the central portion of the plate thickness, and is in the range of 5 to 30 ° C./s. When the average cooling rate is less than 5 ° C./s, the structure is mainly composed of polygonal ferrite, and it becomes difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, when the average cooling rate exceeds 30 ° C./s, the concentration of the alloy elements into the untransformed austenite becomes insufficient, and the desired amount of massive martensite cannot be finely dispersed by the subsequent cooling. It is difficult to obtain a hot-rolled steel sheet having a low yield ratio and a desired excellent low-temperature toughness. Further, when the average cooling rate exceeds 30 ° C./s, the surface layer portion has a martensite single phase structure, and is subsequently tempered to become a tempered martensite single phase structure, resulting in a high yield ratio. For this reason, the cooling rate after finishing rolling was limited to the range of 5 to 30 ° C./s on average. In addition, Preferably it is 5-25 degrees C / s.

The above-described cooling stop temperature is set to a temperature in the range of 600 to 450 ° C. When the cooling stop temperature is higher than the above temperature range, it is difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, when the cooling stop temperature is lower than the above temperature range, the untransformed γ is almost completely transformed and a desired amount of massive martensite cannot be secured.
In the present invention, following the above-described primary cooling, as the secondary cooling, the cooling in the temperature range from the above-described cooling stop temperature to the winding temperature is set to slow cooling as schematically shown in FIG. By slowly cooling this temperature range, alloy elements such as C are further diffused into the untransformed γ, the untransformed γ is stabilized, and the subsequent cooling facilitates the formation of massive martensite. As such slow cooling, the above cooling stop temperature to the winding temperature are averagely cooled at a cooling rate of 2 ° C./s or less, preferably 1.5 ° C./s or less, or from the above cooling stop temperature. Cooling is allowed to stay for 20 s or more in the temperature range up to the taking temperature. When cooling from the cooling stop temperature to the coiling temperature on average at a cooling rate exceeding 2 ° C./s, alloy elements such as C cannot be sufficiently diffused into the untransformed γ, and the stabilization of the untransformed γ is insufficient. Thus, as in the cooling indicated by the dotted line in FIG. 1, the untransformed γ remains in the form of a barite ferrite and becomes rod-shaped, making it difficult to produce desired massive martensite.

In addition, it is preferable to perform this secondary cooling by stopping water injection in the latter stage of the run-out table. In the case of a steel plate having a thin plate thickness, it is preferable to adjust by completely removing the cooling water remaining on the steel plate, installing a heat insulating cover, or the like in order to ensure desired cooling conditions. Furthermore, in order to ensure a residence time of 20 seconds or more in the above temperature range, it is preferable to adjust the conveyance speed.
After the secondary cooling, the hot rolled steel sheet is subjected to a winding process.

The winding step is a step of winding at a surface temperature and a winding temperature of 450 ° C. or higher.
If the coiling temperature is less than 450 ° C., the desired low yield ratio cannot be realized. For this reason, the coiling temperature was limited to 450 ° C. or higher. By setting it as the above-mentioned process, it can be made to retain for a predetermined time or more in the temperature range in which ferrite and austenite coexist.
A hot-rolled steel sheet manufactured by the above-described manufacturing method is used as a pipe-forming material, and is subjected to a normal pipe-making process to be a spiral steel pipe or an electric-welded steel pipe. The pipe making process is not particularly limited, and any ordinary process can be applied.

  Hereinafter, the present invention will be described in more detail based on examples.

  The molten steel having the composition shown in Table 1 was made into a slab (thickness: 220 mm) by a continuous casting method to obtain a steel material. Subsequently, these steel materials are heated to the heating temperature shown in Table 2 and subjected to rough rolling to form a sheet bar, and then the sheet bar is subjected to finish rolling under the conditions shown in Table 2 to obtain a hot-rolled steel sheet (sheet thickness: 7.8-25.4 mm) was subjected to a hot rolling process. The obtained hot-rolled steel sheet is cooled immediately (within the time shown in Table 2) after finishing rolling, and cooled to the cooling stop temperature (winding temperature) shown in Table 2 at the average cooling rate shown in Table 2. The primary cooling to be performed and the cooling process to perform the secondary cooling under the conditions shown in Table 2 were performed. After the cooling step, a winding step of winding at a winding temperature shown in Table 2 and then allowing to cool was performed.

Test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and impact test. The test method is as follows.
(1) Microstructure observation From the obtained hot-rolled steel sheet, a microstructural specimen was taken so that the cross section in the rolling direction (L cross section) became the observation surface. The specimen is polished, subjected to Nital corrosion, and the structure is observed using an optical microscope (magnification: 500 times) or an electron microscope (magnification: 2000 times). The type, the fraction of each phase (area ratio), and the average particle size were measured. The average particle size of bainitic ferrite as the main phase was determined by a cutting method in accordance with JIS G 0552. The aspect ratio of the martensite grain is calculated by the ratio of the length in the longitudinal direction (long side) and the length in the direction perpendicular to the grain (short side) (long side) / (short side). It shall be. Martensite grains having an aspect ratio of less than 5.0 are defined as massive martensite. Martensite having an aspect ratio of 5.0 or more is referred to as “bar-shaped” martensite. The size of the massive martensite is 1/2 the sum of the long side length and the short side length of each grain of the massive martensite, and the diameter of each obtained grain is arithmetically averaged. The average size of martensite was used. In addition, the largest value among the diameter of each grain of massive martensite was made into the maximum of the magnitude | size of massive martensite. The measured martensite grains were 100 or more.
(2) Tensile test From the obtained hot-rolled steel sheet, the tensile test pieces (all specified in API-5L) were adjusted so that the tensile direction was a direction perpendicular to the rolling direction (sheet width direction) and 30 ° direction from the rolling direction. Thickness specimens (GL 50 mm, width 38.1 mm) were collected and subjected to a tensile test according to ASTM A 370 to determine tensile properties (yield strength YS, tensile strength TS).
(3) Impact test V-notch test specimens were taken from the obtained hot-rolled steel sheet so that the longitudinal direction of the test specimen was perpendicular to the rolling direction, and Charpy impact test was performed in accordance with ASTM A 370 regulations. And the fracture surface transition temperature vTrs (° C.) was determined.

The obtained results are shown in Table 3.
In addition, a spiral steel pipe (outer diameter: 1067 mmφ) was manufactured by a spiral pipe making process using the obtained hot-rolled steel sheet as a pipe material. From the obtained steel pipe, take a tensile test piece (test piece specified in API) so that the tensile direction is the pipe circumferential direction, conduct a tensile test in accordance with the provisions of ASTM A 370, and obtain tensile properties ( Yield strength YS, tensile strength TS) were measured. From the obtained results, ΔYS (= steel pipe YS−steel sheet YS) was calculated, and the degree of strength reduction due to pipe making was evaluated.
The obtained results are also shown in Table 3.

  In all of the examples of the present invention, no special heat treatment was performed, the yield strength in the direction of 30 ° from the rolling direction was 480 MPa or more, the tensile strength in the sheet width direction was 600 MPa or more, and the fracture surface transition temperature vTrs was − It is a low yield ratio high strength high toughness hot-rolled steel sheet having a low yield ratio of 80 ° C. or less and a yield ratio of 85% or less. On the other hand, the comparative example out of the scope of the present invention is desired if the yield strength is insufficient, the tensile strength is reduced, the toughness is reduced, or the low yield ratio is not secured. A hot-rolled steel sheet having the following characteristics has not been obtained.

  Furthermore, all of the examples of the present invention, after being piped to become a steel pipe, are not susceptible to a decrease in strength due to the pipe making, and are suitable hot rolled steel sheets as materials for spiral steel pipes or ERW steel pipes.

Claims (9)

% By mass
C: 0.03-0.10%, Si: 0.10-0.50%,
Mn: 1.4-2.2%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
Nb: 0.02-0.10%, Ti: 0.001-0.030%,
Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%,
Ni: 0.01-0.50%
And a composition comprising the balance Fe and inevitable impurities, and a structure containing bainitic ferrite as a main phase and a bulk martensite with an aspect ratio of less than 5.0 as an area ratio of 1.4 to 15% as a second phase. And an average grain size of the bainitic ferrite is 10 μm or less, a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness. The low-yield-ratio high-strength hot-rolled steel sheet according to claim 1, wherein the composition is a composition satisfying a range of 1.4 to 2.2% by mass% and Moeq defined by the following formula (1): .
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
Here, Mn, Ni, Cr, Mo: Content of each element (mass%)   In addition to the composition, the composition further contains one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less in terms of mass%. The low yield ratio high strength hot rolled steel sheet according to 1 or 2.   The low yield ratio and high strength hot-rolled steel sheet according to any one of claims 1 to 3, further comprising Ca: 0.0005 to 0.0050% by mass% in addition to the composition.   5. The low yield ratio high strength hot-rolled steel sheet according to claim 1, wherein the bulk martensite has a maximum size of 5 μm or less and an average of 0.5 to 3.0 μm. The steel material is subjected to a hot rolling process, a cooling process, and a winding process to make a hot rolled steel sheet.
The steel material in mass%
C: 0.03-0.10%, Si: 0.10-0.50%,
Mn: 1.4-2.2%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
Nb: 0.02-0.10%, Ti: 0.001-0.030%,
Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%,
Ni: 0.01-0.50%
And a steel material having a composition consisting of the balance Fe and inevitable impurities,
In the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C., the heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less. Cumulative rolling reduction: It is a process to make a hot-rolled steel sheet by applying finish rolling to 50% or more,
The cooling process starts cooling immediately after finishing rolling, and performs primary cooling to cool to a cooling stop temperature in the temperature range of 600 to 450 ° C. at 5 to 30 ° C./s at an average cooling rate at the center of the plate thickness. Further, as the secondary cooling, the cooling from the cooling stop temperature to the winding temperature is cooled at an average cooling rate of 2 ° C./s or less, or the temperature is kept for 20 s or more in the temperature range from the cooling stop temperature to the winding temperature. Process and
The winding step is a step of winding at a winding temperature of 450 ° C. or more at the surface temperature;
A method for producing a high-strength hot-rolled steel sheet having a low yield ratio and excellent in low-temperature toughness. The low-yield-ratio high-strength hot-rolled steel sheet according to claim 6, wherein the composition is a composition satisfying a mass% and a Moeq defined by the following formula (1) in a range of 1.4 to 2.2%. Manufacturing method.
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
Here, Mn, Ni, Cr, Mo: Content of each element (mass%)   In addition to the composition, the composition further contains one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less in terms of mass%. A method for producing a high yield hot rolled steel sheet having a low yield ratio according to 6 or 7.   The method for producing a low yield ratio high strength hot-rolled steel sheet according to any one of claims 6 to 8, further comprising Ca: 0.0005 to 0.0050% by mass% in addition to the composition.

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